A. Overall objectives: 1. To clarify the cellular and molecular mechanisms which determine and regulate hematopoietic and stem cell differentiation. 2. To clarify the relationship between differentiation and malignancy. B. Species studied: Mice and humans C. Summary: We have been studying the role of nuclear oncogenes in growth and differentiation. Much of our work has focused on c-myb which encodes a nuclear, DNA-binding protein. We have cloned and sequenced the mouse c-myb mRNA, and have defined the major transcription unit. The finding of 5' mRNA heterogeneity and different size forms of mouse c-myb protein suggests alternative 5' RNA splicing, although we have no definitive information to clarify this situation presently. Preliminary results indicate that the major mechanism by which c-myb expression is regulated in mouse hematopoietic cells is modulation of transcription elongation occurring at a site in intron 1. We have demonstrated that pre-B cell lines express 10- 100 times as much c-myb mRNA as more mature B cells. This finding has helped us to demonstrate that pre-B cell lines which begin to express surface immunoglobulin retain a pre-B cell phenotype. Finally, we have established that down-regulation of c-myc mRNA is necessary for mouse erythroleukemia cells to undergo terminal differentiation; preliminary results suggest that down-regulation of c-myb mRNA is also necessary for terminal differentiation. These studies suggest that constitutive expression of normal proto-oncogenes may account for the observation that tumor cells are generally "frozen" in a particular state of differentiation.